The present invention relates to a developing device and an image forming device having the developing device.
A conventional image forming device of an electric photography type such as a printer, a copier, and a facsimile forms an image through an electric photography process. Such an image forming device includes a charging roller for uniformly charging a surface of a photosensitive drum; an exposure unit for exposing the surface of the photosensitive drum to form a static latent image thereon; a developing roller for attaching toner to the static latent image to form a toner image; a toner supply roller for charging and supplying toner to the developing roller; a developing blade for forming a toner layer with a uniform thickness on a surface of the developing roller; a transfer roller for transferring the toner image on the surface of the photosensitive drum to a sheet; and a cleaning device for collecting toner not transferred and remaining on the surface of the photosensitive drum.
Japanese Patent Publication No. 2001-305856 has disclosed a developing blade. When a toner layer is formed on a surface of a developing roller, the developing blade scrapes excess toner on the surface of the developing roller, thereby making a thickness of the toner layer uniform. In Japanese Patent Publication No. 2001-305856, the developing blade is formed of an elastic plate member for easily scraping toner, and a curved portion of the developing blade is pressed against the developing roller.
The curved portion of the developing blade is pressed against the developing roller at a downstream side in a direction that the developing roller rotates relative to a top of the curved portion of the developing blade. The top of the curved portion of the developing blade has a minimum curvature radius, and the curvature radius increases toward a downstream side in a direction that the developing roller rotates. That is, when Rc represents the curvature radius at the top of the curved portion of the developing blade and R represents the curvature radius at the abutting point where the developing blade abuts against the developing roller, the following relation is established.Rc≦R
It is possible to adjust a thickness of the toner layer on the surface of the developing roller through the curvature radius of the abutting point. In the image forming device disclosed in Japanese Patent Publication No. 2001-305856, the toner tends to expel each other due to a charged state of the toner. Accordingly, the developing blade is lifted through an expel force, thereby shifting the abutting point between the developing blade and the developing roller. As described above, the top of the curved portion of the developing blade has a minimum curvature radius, and the curvature radius increases toward the downstream side in a direction that the developing roller rotates. Therefore, when the abutting point is shifted, the curvature radius R at the abutting point is also changed. As a result, the thickness of the toner on the surface of the developing roller changes, thereby making it difficult to stably form an image.
In view of the problems described above, an object of the present invention is to provide a developing device and an image forming device for forming a toner layer with a uniform thickness on a surface of a developing roller, so that an image is stably formed even when an abutting point between a developing blade and the developing roller is shifted.
Further objects and advantages of the invention will be apparent from the following description of the invention.